Harnessing a river without a dam
Before 256 BCE, the Min River, a major tributary of the Yangtze, would rush down from the mountains and flood the Chengdu Plain annually. The governor of Shu province, Li Bing, was tasked with controlling the floods but also had to keep the river open for military vessels. A simple dam was not an option. His solution was an elegant system that works with the river's natural forces to divide, channel, and desilt the water. Today, it irrigates an area of over 5,300 square kilometers.
The engineering works have three core components that passively manage the powerful river. This system, a UNESCO World Heritage Site, relies entirely on gravity and the physics of water flow. Construction began around 256 BCE under the direction of Li Bing and his son, employing a workforce of tens of thousands.
A three-part fluid dynamics solution
The first component is the Yuzui, or "Fish Mouth Levee," a cone-shaped dike built in the middle of the river. Its position at a natural bend divides the Min River into an Inner and Outer Stream. During the dry season, the Inner Stream receives about 60% of the water for irrigation, while in the flood season, this automatically drops to 40%, diverting the majority of the excess water into the Outer Stream. This passive regulation is achieved by the levee's shape and its interaction with the river's natural curve.
The second piece is the Feishayan, or "Flying Sand Weir." This is a low spillway, about 200 meters wide, connecting the Inner and Outer streams. During high water events, it allows excess water to escape from the Inner Stream, preventing floods. More importantly, it exploits vortex flow; the natural swirl of the water efficiently ejects about 80% of the silt and sediment from the Inner Stream over the weir and into the Outer Stream, preventing the irrigation channels from clogging.
The final component is the Baopingkou, or "Bottle-Neck Channel," which is the primary intake for the irrigation network. This channel was gouged through Mount Yulei, a project that took eight years. Lacking gunpowder, workers heated the rock with fire and then doused it with cold water, causing it to crack and break apart. The channel is strategically narrow, just 20 meters wide, which helps control the volume of water entering the irrigation plains and creates a swirling flow that further helps to remove sediment.